The present invention relates to the tuning of integrated LC filters, and more particularly to the tuning of integrated LC filters in broadband television receivers.
Modern receiver systems utilize broadband technology to receive a wide range of frequencies. In order to prevent in-band blockers from interfering with the receiver operation, a receiver requires very high linearity at its front end. In broadband television receivers, a conventional method of achieving high linearity in the presence of in-band blockers is to utilize Automatic Gain Control (AGC). A desired signal together with the in-band blocker are applied to a conventional receiver and amplified utilizing a broadband Low Noise Amplifier (LNA) and a Variable Gain Amplifier (VGA). A Wide Band Power Detector, coupled to the output of the VGA, converts the desired signal and blocker power to a DC voltage, which is then input to the base-band part of the receiver. As the DC voltage is directly proportional to the input signal power, the base-band is able to detect the presence of strong blocker(s) if the corresponding base-band Radio Signal Strength Indicator (RSSI) level is not proportional to the WBPD DC level. When this occurs, the base-band will send a signal to the front end to decrease the gain of the VGA. As the gain decreases, however, the receiver noise figure will also increase. Therefore, the use of AGC is limited.
Many receivers therefore utilize tracking filters to amplify only the desired input signal, thus reducing the need for such high linearity. In a direct conversion receiver the tracking filter can track the internal Local Oscillation (LO) signal generated by a phase locked loop (PLL). The frequency response of a filter refers to the characteristic(s) of the filter that conditions the input/internal signal to the filter. The filter will show frequency responses based on certain circuit parameters.
In a broadband receiver, a filter is required that has many frequency responses, i.e. a tunable filter. Prior art receivers utilize radio frequency (RF) tracking filters, which are equivalent to band-pass filters. U.S. Pat. No. 6,285,865, which is included here for reference, discloses a receiver with such a tunable filter. The disclosed integrated chip comprises: a first adjustable on-chip filter having a first plurality of selectable capacitors that determine its center frequency; a second adjustable on-chip filter having a second plurality of selectable capacitors that determine its center frequency; means for selecting a number of the first plurality of capacitors to adjust the first filter to a desired center frequency; and means for transferring the selection of the first plurality of capacitors to the second plurality of capacitors to adjust the second filter to a center frequency proportional to the desired frequency. The receiver described needs to calibrate a dummy tunable filter first before transferring the results from the dummy filter to the other tunable filter in the main signal path. The need to duplicate and tune another tunable filter is partly due to the fact the tuning of the filter in the main signal path will be affected by interferences coming from the antenna. This causes the die area to increase unnecessarily.
Recently, other receivers incorporating integrated tracking filters have been developed. U.S. Pat. No. 7,127,217 is included as an illustration, and shown in FIG. 4. In this prior art, the entire receiving path including the down-mixers must be configured to receive the filtered signal from the tunable filter. In addition, the calibration signal will radiate through the antenna if an additional antenna switch is not present.
In both prior arts, the calibration of these tunable filters must either be done in the factory to prevent radiation through the antenna, or outside the signal path. A receiver comprising an integrated tracking filter that minimizes interference to the receiver and prevents the radiation of calibration signal into the air during the filter calibration process is needed. If the entire receiver path is not required during the tuning process, receiver power consumption may be reduced even further.